Cleaning and pickling of metals



Patented Aug. 12, 1947 UNITED STATES PATENT OFFICE Serial No. 463,133

23 Claims.

This invention relates to improvements in pickling and cleaning of metals and in inhibiting or retarding the action of acids on metals. More particularly the invention relates to improvements in inhibitors for acids, solutions containing these inhibitors for cleaning or pickling metals, and methods of cleaning or pickling metals and removing oxides or scale therefrom.

Such metallic substances as ferruginous metals, aluminum, zinc, nickel and others are, as is well known, subject to surface corrosion when exposed to the atmosphere and certain chemicals. Metal surfaces that are thus affected and that have become coated with scale, particularly mill scale formed in sheet rolling or other operations, rust and the like are generally treated with a pickling solution to remove the objectionable material. The pickling solution generally contains an acid, and a so-called inhibitor that is added for the purpose of preventing dissolution of the metal without interfering with the dissolution action of the acid on the objectionable material.

It is of considerable advantage to employ as inhibitors, substances that have a high solubility in pickling acid solutions, that are readily wetted by acids, and that, by rinsing with water, are easily and completely washable from metal along with carbon smut and other debris. It is also an advantage to employ compounds that do not have a disagreeable odor and that are not irritating to the skin and mucous membranes of workmen.

The above and other advantages are gained by the following invention in which a pickling solution is provided by adding to an acid solution a pr9duct obtained by chemical reaction between an aldehyde, certain nitrogen "compounds and a thiocyanic acid compound; The condensation products formed include in their molecular structure both sulfur and nitrogen thereby rendering them particularly effective as inhibitors.

The nitrogen compounds from which the thiocyanates are prepared are basic amin ggomof either the aliphaticoi stfiight chain, ca rbocyclic, or heterocyclic itmes. They contain one or more a he or e groups, or tertiary nitrogen atoms, or combinations thereof. It is to be understood in the present instance that the term amino compound as herein employed, applies to compounds containing primary, secondary and tertiary amine structures in their configuration and wherein one, two or three atoms of ammoniacal hydrogen have been replaced by other linkages.

Decided benefits are obtained-from increasing the number of amine or imine groups in a compound as a starting material. .argridinesand p lyaiines, for instance, are generally found to be particularly satisfactory. Furthermore, by using as starting materials basic organic compounds which will in themselves resinify with aldehydes, as for example, amidines or compounds capable of forming Schiffs bases on condensation with aldehydes, a firmer anchoring of the nitrogen and sulfur atoms in the molecule is obtained.

Under steel plant working conditions it is desirable to use compounds that are thermostable. It is noted that in general the presence of alkyl, alkoxy, aralkyl, and especially aryl groups in the amino starting compounds prov es creased stability in the inhibitor. For instance, the dipheny] guanidmte aldehyde condensation product; is more thermostable than the corresponding condensation product made from guanidine thiocyanate.

The stronger the basic characteristics of the amino compound used as a starting material the better the compound is for purposes of the present invention. It is preferred that the amino compound be basic enough to form with thiocyanic acid true thiocyanates of sufiicient stability to avoid continuous splitting off of free thiocyanic acid in too great proportions during condensation. The bases employed should preferably have a, basicity greater than that expressed by the dissociation constant 10 The presence of water solubilizing groups, such as NHz, NH, 0H and others, in the amino compound or in the inhibitor product confers upon the inhibitor increased solubility in the acid baths in which it is used.

The following are some of the amino compounds which have been found to be useful in providing the acid inhibitors of the present invention.

Guanidine and its derivatives and substitution products such as methyl and dimethyl guanidine, ethyl and diethyl guanidine, butyl and dibutyl guanidine, octyl and dioctyl guanidine, diethanol guanidine, dicyclohexyl guanidine, dibenzyl guanidine, diphenyl guanidine, ditolyl guanidine,

, dixylyl guanidine, dinaphthyl guanidine; bi-

uanid and its derivatives and substitution products such as ethyl and diethyl biguanid, propyl and dipropyl biguanid, octyl and dioctyl biguanid, mono-, di-, and triethanol biguanid, dimethylclohexyl biguanid, dibenzyl biguanid, phenyl and diphenyl biguanid, tolyl biguanid, morpholino biguanid; guanyl urea and its derivatives and substitution products such as dimethylguanyl urea, diethanol guanyl urea, diphenylguanyl urea; cyclic amidines such as melamine, formoguanamine, acetoguanamine, melam, melem, 2,4,6-trihydrazino-1,3,5-triazine; monoand polyamines such as dimethylamine, monoethylamine, monon-propylamine,"di isopropylamine, tri-n-butylamine, mono-n-butyl diamylamine, dioctylamine,

mono-n-dodecylamine, mono-n-octadecylamine, benzyl diethylamine, aniline. p;tertiary-amylaniline, di-tertiary amylaiiiline, N-monoamylaniline, N,N-diamyl aniline, N-mono-n-butyl aniline, N,N-di-n-butylaniline, a-naphthylamine, ,B-naphthylamine, N-n-butyl-e-naphthylamine, o-amino diphenyl, diphenylamine, piperidine, morpholine, phenylmorpholine, phenylmethyl pyrazolone, monoethanolamine, diethanolamine, ethyldiethanolamine, n-butyl monoethanolamine, 2-amino-1-butano1, 2 amino-2-ethyl 1,3 propanediol, tris (hydroxymethyl) aminomethane, phenylethanolamine, p-tertiary-amyl phenyl diethanolamine, ethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pentamine, 2,3-diamino-2,3-dimethylbutane, pentamethylene diamine, hexamethylene tetramine, pphenylene diamine, benzidine.

The above amino compounds, either singly or mixtures of two or more thereof, are readily converted to thiocyanates or perthiocyanates by reaction with thiocyanic acid or a perthiocyanic acid. or by the double decomposition of an amine salt and a metal or ammonium thiocyanate.

For the aldehyde component of the new compounds there may be used for example formaldehyde, acetaldehyde, butyraldehyde, crotonaldehyde, acrolein, benzaldehyde, salicylaldehyde, cinnamic aldehyde, furfural, glyoxal, and others and mixtures thereof. Formaldehyde is in, general to be preferred on account of its availability, cheapness, and effectiveness.

As to the relative proportion of amino compound and aldehyde to be used, no general rule can be laid down. The ratio depends on the number of active groups in the amino compound molecule, such as NH2 and thiocyanate groups. One mol of aldehyde is usually employed for each reactive group, and therefore in most cases two mols of aldehyde are suflicient for a. particular thiocyanate. However, an excess of for example formaldehyde does no harm, since it is in itself a pickling inhibitor of some limited value and may therefore be left with the reaction product in many cases. A deficiency of aldehyde also is of no grave consequence since practically all amines and all thiocyanates in a sense have some inhibiting value of their own. Where desired, an excess of aldehyde may be removed by means known to the art, such as evaporation in vacuo or otherwise.

When using the pickling inhibitors of the present invention, it is often advantageous to add wetting agents to the pickling baths or to incorporate such agents with the inhibitors themselves. Such commercial wetting agents as a naphthalene sulphonic acidtype of wetting agent, known as .fl erosol .OS. render good service. I'hey'should be employed in quantities of about /z% of the acid weight of the bath. The addition of wetting agents facilitate the removal of carbon smut from pickled steel or other metal goods. At the same time, though only very small amounts of the inhibitors are needed in a pickling bath in order to inhibit effectively the solvent action of the acid in the bath on the metal as seen in the following examples, the addition of the wetting agent increases the effectiveness of the inhibitors to such an extent that substantially smaller percentages of the inhibitors give satisfactory results.

Examples 1. Approximately 118 parts by weight of guanidine thiocyanate are mixed with about 75 parts by weight of 40% formaldehyde, and the mixture is left standing at room temperature for three hours. Then it is simmered for about thirty minutes under reflux and allowed to stand. The product is a yellow liquid. This was tested as a pickling inhibitor by adding a drop of it to a solution of 25 cc. concentrated hydrochloric acid in 50 c. c. water, and dropping into the solution 2 lath nails. The compound dissolved in the acid easily and without residue. Parallel tests were nm on the same acid concentration and the same kind of nails, but without the addition of the inhibitor product in one case and with a drop of formalin solution in the other case. The inhibition effect brought about by the addition of the guanidine thiocyanate-formaldehyde condensation product was strikingly seen from the lessened attack of the acid on the nails; even after a week the nail surfaces were very little affected.

The product described in Example 1 was evaluated quantitatively in comparison with a commercial inhibitor under conditions simulating steel plant practice. The weight loss of steel plate of known area was determined after pickling periods of 10, 20, and 30 minutes. The acid bath used contained 5% by volume of sulfuric acid and was kept at 180 F. Bessemer steel plate was used for the tests, and the weight losses given in columns (1), (2) and (3) for the inhibitor of Example 1, and in column (4) for the known commercial inhibitor, were determined in ounces per square foot of surface. The following table includes the results obtained in these tests:

Quanidine Length of Immersion thiocyanate- Commercial Length formaldehyde Inhibitor of Imoond. prod- Concentramersion uct coneen- 10 min. 20 mm. 30 min. tion (per 30 min. tration (per cent of acid cent of acid wt.)

ig t) (1) (4) 2. About 118 parts by weight of guanidine thiocyanate are added to about parts by weight of 40% formaldehyde solution, and the resulting mixture is allowed to stand at room temperature for about three hours. In that time most of the thiocyanate dissolves. The mixture is simmered for about hour under reflux and permitted to stand. The resultant yellow liquid was tested as in Example 1 and found to be substantially as effective as the product used in Example 1.

3. Biguanid thiocyanate having the empirical formula C2N5H'I.HSCN is prepared from biguanid neutral sulfate and barium thiocyanate. About 80 parts of the resulting crystalline compound are dissolved in about 80 parts of formaldehyde solution of 40%. The mixture is gently boiled in an open vessel until a frothy mass is obtained, which is waterwhite and free of odor. The resulting mass, which is a thick liquid, was tested out for its pickling inhibition value as in Example 1 and compound described is gently boiled in an open vessel. As soon as excess formaldehyde disappears heating is discontinued. The product is a clear, thick, oily liquid of little odor, and dissolves in acid speedily. It was tested as a pickling inhibitor as in Example 1 and found to be very effective.

5. Dicyandiamidine thiocyanate formaldehyde condensation product is prepared as follows: about 320 parts by weight of dicyandiamidine thiocyanate are refluxed for about hour with about 300 parts by weight of 40% aqueous formaldehyde. A clear, waterwhite solution results which gradually thickens and becomes a little yellowish. On cooling, the product sets to a somewhat translucent gel containing some thick liquid which curdles on addition of water. The gel melts at 240 to 250 C. with rapid decomposition to a brown froth. The gel on testing according to Example 1 was found to be a very excellent pickling inhibitor.

6. 2'70 parts by weight of diphenylguanidine thiocyanate are mixed with 225 parts by weight of 40% aqueous formaldehyde and gently heated in an open vessel. A slightly pinkish solution results which quickly turns colorless, then yellow. The heating is discontinued as soon as the formaldehyde odor disappears. on cooling, the new product solidifies to a light-yellow, clear, transparent resin. When at room temperature, the resin is hard and tough. When tested out for its pickling inhibition value as in Example 1 it was found to be extremely good.

'7. 210 parts by weight of diphenylguanidine thiocyanate and 150 parts by weight of 40% aqueous formaldehyde are heated under reflux for two hours. The product is then heated in an open vessel on a steam bath for six hours, and thereafter on a low hot plate (asbestos pad) for one hour more. The final material is a clear, faintly yellow. somewhat brittle resin having a melting point of 80 to 90 C. It was found to be a good pickling inhibitor when tested according to the procedure outlined in Example 1.

Comparative tests under steel mill conditions with two of the best known marketed commercial inhibitors proved the product of Example '7 to be superior to either of the commercial inhibitors as seen from the following tabulation of results obtained in tests in which open hearth and Bessemer steel plate respectively were immersed for meriods of 10, 20, and 30 minutes in acid baths containing 5% by volume of sulfuric acid and kept at 180 F. The weight losses were determined in ounces per square foot of surface. Weight losses were also established for identical acid baths which contained commercial inhibitors A and R.

Commercial Inhibitor 30 minutes 30 minutes A (per cent of acid Immersion, (per cent of immersion, weight) wt. losses acid weight) wt. losses OPEN HEARTH PLATE BESSEMER PLATE 8. 250 parts by weight of o-tolyl biguanid thiocyanate and 300 parts by weight of 40% aqueous formaldehyde are refluxed for. 3-hours. -.The resultant product is poured out in a thin layer and on cooling turns to a white, opaque resin. After exposure to the room atmosphere for a week, it loses the water derived from the aqueous formaldehyde, and which causes the opaqueness. It is then transparent, water-white, and brittle enough to be coarsely ground. The melting point is about 70 to C. Its pickling inhibiting eifect was tested out as in Example 1 and found to be excellent, probably aided by the ease with which it dissolves in the acid.

9. 185 parts by weight of mixed triazine thiccyanates containing the thiocyanates of melamine, melam, melem, and melon and obtained by thermal decomposition of ammonium thiccyanate, are reacted with 225 parts of 40% aqueous formaldehyde by heating the mixture to just under the boiling point. On cooling, the turbid, light tan solution sets to a solid, which is soluble in acid and does not melt at temperatures up to 300 C. but commences to turn brown at about 260 C. It proved to be a good pickling inhibitor when tested according to the scheme used in Example 1.

10. 1940 c. c. of an aqueous solution of aniline thiocyanate containing 3.3 mols of the compound, are slowly added to 495 grams (6.6 mols) of a 40% aqueous formaldehyde solution. The mixture is constantly stirred and kept at about 12 C. Slowly a milkiness begins to appear and the color gradually changes to a light yellow due to finely dispersed solid. The color of the solid deepens while the amount increases, until a large quantity of a scarlet colored, finely divided product is produced at the end of the addition of the aniline thiocyanate solution. Filtration and washing with water yields an almost theoretical quantity of the desired anilin thiocyanate formaldehyde condensation product. The new material is a thermosetting resin which does not melt at temperatures up to 300 C. and on heating gradually turns brown and than black. Tested by the procedure of Example 1, it was found to be a good pickling inhibitor.

11. 106 parts by weight of diphenylguanidine thiocyanate are refluxed with 44 parts by weight of acetaldehyde. A liquid is formed which gradually turns dark brown. After two hours refluxing, the product amounts to 116 parts by weight. Thus, a considerable part of the aldehyde evaporates due to its low boiling point. The product is a very dark brown solid with an odor of heterocyclic nitrogenous bases. It is easily soluble in acids and has a melting point of 75 to C. It proved to be a good inhibitor when tested according to the method described in Example 1.

'irT i-s 12. 270 parts by weight of diphenylguanidine thiocyanate are refluxed with '10 parts by weight of croton aldehyde for one hour. The reaction product is poured out and solidified to a clear reddish resin which after a few days exposure is brittle enough to be broken up. It has a melting point of 85 to tested for its inhibiting efiect according to the procedure in Example 1, it was found to be a very good inhibitor.

13. 250 parts by weight of diphenylguanidine thiocyanate are refluxed with 120 parts by weight of butyraldehyde for one hour. The product is a golden-yellow oil which slowly crystallizes to a solid of corresponding color having a melting point of 100 to 105 C. Tested by the procedure of Example 1, it proved to be an excellent pickling inhibitor.

14. 230 parts of 2-amino-2-methyl-l-propanol thiocyanate are refluxed with 380 parts of 40% aqueous formaldehyde for six hours. The resultant product is an amber, clear liquid, 9. little bydrophobic but substantially water soluble. It is entirely soluble in trated hydrochloric acid and two parts water. Tests prove it to be an excellent pickling inhibitor.

15. 300 parts of 2-amino-2-ethy1-1,3-propanediol thiocyanate are refluxed with 380 parts of 40% aqueous formaldehyde for six hours. The resultant product is a dark, red-brown, clear liquid rather hydrophobic. Tests prove it to be entirely soluble in a mixture of one part concentrated hydrochloric acid and two parts water. It is a most excellent inhibitor.

16. 230 parts of Z-amino-l-butanol thiocyanate are refluxed with 380 parts of 40% aqueous formaldehyde for six hours. The resultant product is a very deep red-brown, thick liquid, quite hydrophobic but completely soluble in a mixture of 1 part concentrated hydrochloric acid and 2 parts water. Tests prove it to be a most excellent pickling inhibitor.

What is claimed is:

1. A cleaning and pickling composition for to atmospheric corrosion and for corrosion products from surfaces of said metal, said composition comprising a pickling acid solution having dissolved therein a relatively small proportion or a reaction product of an aldehyde and a preformed thiocyanate of a basic, nitrogenous organic comleast one nitrogen-containing radical selected from the group consisting of NH: and NH, said reaction product being soluble in said pickling acid solution in a proportion which inhibits dissolution of said metal by said acid.

2. A cleaning and pickling composition for metal subject to atmospheric corrosion and for removing oxides and other corrosion products from surfaces of said metal, said composition comprising a pickling acid solution having dissolved therein a relatively small proportion of a reaction product of an aldehyde and a preformed thiocyanate of a basic amino compound, said reaction product being soluble in said pickling acid solution in a proportion which inhibits dissolution of said metal by said acid.

3. The composition of claim 2 in which the aldehyde is formaldehyde.

4. The composition of claim 2 in which the basic amino compound is an amidine.

5. The composition of claim 2 in which the thiocyanate is normal thiocyanate.

6. The composition of claim 5 in which the basic amino compound is a guanidine.

7. A cleaning and pickling composition for metal subject to atmospheric corrosion and for removing oxides prising a pickling acid solution having dissolved therein a relatively small proportion of a reaction product of an aldehyde and preformed normal thiocyanate of dlphenyl guanidine, said reaction product being soluble in said pickling acid solution in a proportion which inhibits dissolution of said metal by said acid. 8. The composition of aldehyde is formaldehyde.

9. A cleaning and pickling composition for metal subject to atmospheric corrosion and for removing oxides and other corrosion products from surfaces of said metal, said composition comprising a pickling acid solution having dissolved therein a relatively small proportion of a reaction product of an aldehyde and preformed normal thiocyanate of dixylyl guanidine, said reaction product being soluble in said pickling acid solution in a proportion which inhibits dissolution of said metal by said acid.

10. The composition of claim aldehyde is formaldehyde.

11. A cleaning and pickling composition for metal subject to atmospheric corrosion and for products claim 7 in which the 9 in which the from comprising a pickling acid solution solved therein a relatively small proportion of a reaction product of an aldehyde and preformed normal thiocyanate of normal guanidine, said reaction product being soluble in said pickling acid solution in a proportion which inhibits dissolution of said metal by said acid.

12. The composition of claim 11 in which the aldehyde is formaldehyde.

13. In a method of cleaning and pickling a metal, the step comprising treating the metal with an aqueous pickling acid solution having dissolved therein a relatively small proportion of a product of reaction between an aldehyde and a preformed thiocyanate of a basic, nitrogenous, organic compound having at least one nitrogencontaining radical selected from the group consisting of N112 and NH to inhibit dissolution of said metal by said pickling acid.

14. In a method of cleaning and pickling a metal, the step comprising treating the metal with an aqueous pickling acid solution in which is dissolved in acid-inhibiting proportions a product of reaction between an aldehyde and a preformed thiocyanate of a basic amino compound to inhibit dissolution ling acid.

15. The method of claim 14 in which the basic amino compound is an amidine.

16. The method of claim 15 in which the thiocyanate is normal thiocyanate.

17. The method of claim 16 in which the basic amino compound is a guanidine.

18. In a method of cleaning and picklin a metal, the step comprising treating the metal with an aqueous pickling acid solution in which is dissolved in acid-inhibiting proportions a product of reaction between an aldehyde and preformed normal thiocyanate of diphenyl guanidine, to inhibit dissolution of said metal by said pickling acid.

19. The method of claim 18 in whch the aldehyde is formaldehyde.

'20. In a method of cleaning and pickling a 5 metal, the step comprising treating the metal of said metal by said pick-- with an aqueous pickling acid solution in which is dissolved in acid-inhibiting proportions a prodnot of reaction between an aldehyde and preformed normal thiocyanate or diiwlyl guanidine, to inhibit dissolution of said metal by said pickling acid.

21. The method of claim 20 in which the aldehyde is formaldehyde.

22. In a method of cleaning and pickling a metal, the step comprising treating the metal 10 with an aqueous pickling acid solution in which is dissolved in acid-inhibiting proportion a product of reaction between an aldehyde and preformed normal thiocyanate of normal guanidine, to inhibit dissolution of said metal by said pickling acid.

23. The method of claim 22 in which the aldehyde is formaldehyde.

WILLIAM H. HILL.

REFERENCES crrnn The following references are of record in the die of this patent:

UNITED STATES PATENTS Number Name Date 1,780,636 Stine Nov. 4, 1930 2,203,649 Felkers June 4, 1940 2,072,003 Lutz Feb. 23, 1937 2,335,452 ScheIling Nov. 30, 1943 2,050,204 Ter Horst Aug, 4, 1936 FOREIGN PATENTS Number Country Date 347,228 Great Britain Apr. 24, 1931 

